In a digital input interface a DC signal from a remote unit arrives over a signal line. The voltage of the DC signal is used to determine whether a digital. “1” or a “0” is to be sent to other subsystems. At its most basic, a Zener diode can be used in series with a resistor and a current detector. If the DC voltage is high enough that it exceeds the breakdown voltage of the Zener diode then a current flows through the circuit, and the current detector indicates that the DC signal is active. If the DC voltage is lower than the breakdown voltage of the Zener diode then no current flows through the circuit, and the lack of current induces the current detector to indicate that the DC signal is inactive.
For example, rail systems usually have a control system for managing trains. The control system receives state information from remote field elements. Some remote field elements provide this information to the control system by setting the DC voltage on a wire leading to the control system. At the control system the voltage on the wire is used to establish the state of the device to which the respective field element is assigned.
As a simple example, a railroad track circuit is given. To manage train traffic, the track is divided in segments called blocks. When a block is occupied by a train, the track circuit detects the presence of the train and signals to the control systems using a DC voltage. At the control system, the voltage on the wire is detected and used to transmit to subsystems a digital indication of the block occupancy. A diagram of such a system is shown in FIG. 1. The track circuits are always constructed in such way that it will signal “low” or 0V if a train is detected and “high” or 24V (for example) if the block is not occupied. The “high” or active state is called “permissive” in this context because in this state the trains are permitted to enter the block. Opposite, the “low” state is called “restrictive” because trains are restricted from entering the track block.
The signaling method based on permissive/restricted concept described for the track circuit is also applied for other system elements such as train and platform doors, rail switches, trip stop mechanisms, etc. In general, the permissive state is always associated with electrical elements/circuits being in an energized state. With this signaling arrangement, failures such as interrupted wires or bad circuit contacts will always result in “low” signals. In such case traffic will be restricted (stopped) and therefore the possible failure will always result in a safe state.
A digital input interface is termed “vital” if 100% certainty is needed in asserting the “permissive” or “1” or “high” state, and by corollary it must be known if the interface is faulty in such a way that the fault may indicate a “permissive” (“1”) state when the input signal signals in fact a “restrictive” (“0”). Digital input interfaces for rail control systems are often vital. In the example given above, it is crucial that the subsystems correctly know the unoccupied state of the block. An incorrect reading resulting from an unknowingly faulty interface can have disastrous consequences, such as allowing another train to enter the block when the control subsystem erroneously interprets an input signal as “permissive” when in fact the input signal is meant to be read as “restrictive”. It is however acceptable from a safety perspective that the digital input interface may fail in such a way that it will indicate a state of “restrictive” when in fact the field element indicates “permissive”. This type of failure is still undesirable because it will cause trains to stop unnecessarily with consequences in delays and revenue, but at least no accidents will happen.
One cause of error is induced noise. Nearby electrical wires can induce an AC signal in the DC signal sent from the remote field element to the interface. For example, the signal line from a field element to the control system in railroad, systems usually lies along a railroad track. Due the distance between the field element and the control system, which is often at a central location, there is a good chance that the signal line will pass near other electrical wires. The induced AC noise can bring the received voltage above the threshold in a periodic manner. This, in conjunction with the read-by-sampling of the input processor can result in an assignment of a “1” as if a valid DC signal were received. An example of this is illustrated in FIG. 2.
Another cause of error is the decay of the threshold to which the DC voltage is compared in order to determine of the input signal corresponds to a “1” or a “0”. This can occur as the characteristics of circuit components change with age or temperature. Manufacturing issues, environmental conditions, or electrical surges may also produce failure in circuits and components. For example, the breakdown voltage of a Zener diode may gradually change with time, or alternatively the reverse leakage current can increase. This can exacerbate the effects of noise, as following such events low magnitude noise may falsely trigger the input circuit into the “high” state.
Yet another possible cause of error is the asymmetry of the input circuit. Common mode noises may be transformed into differential mode noises, contributing to false triggering of the input circuit into the “high” state.
An interface which minimized the effect of noise would contribute to the vitality of the interface, as would periodic test for detection of threshold decay and noise attenuation capability.